The NFL sent a memo Monday morning to reinforce its requirement that coaches wear face coverings at all times on the sidelines during games, threatening discipline for those who don't comply.

The sharply worded message, written by executive vice president of football operations Troy Vincent, came in response to wide variance in compliance during the first Sunday of the 2020 season. It was directed particularly at head coaches, on whom TV broadcasts frequently focus.

The discrepancy was particularly noticeable Sunday night in Los Angeles, where Rams coach Sean McVay often had his mask pulled down below his chin. Cowboys coach Mike McCarthy wore his mask throughout the game.

Vincent wrote that Week 1 was "tremendous" but noted that "we must remain vigilant and disciplined in following the processes and protocols put in place by not only the league, union and clubs, but also by state and local governments."

Vincent added: "The NFL-NFLPA gameday protocol, which reflects the advice of infectious disease experts, club medical staffs and local and state governmental regulations, requires all individuals with bench area access [including coaches and members of the club medical staff] to wear face coverings at all times.

"Failure to adhere to this requirement will result in accountability measures being imposed against offending individuals and/or clubs. The face covering must be worn as designed so that it securely fits across the wearer's nose and mouth to prevent the transmission of the virus."

The league protocol, updated shortly before the season began, says that anyone with bench area access -- except players -- must wear masks at all times. Coaches are allowed the option of a mask, neck gaiter and/or face shield. Kansas City Chiefs coach Andy Reid chose a face shield for Thursday's game against the Houston Texans, but it fogged up throughout.